1. Field of the Invention
The present invention relates to a method of producing an ultra-low-carbon steel which can produce, by using a vacuum degasifier, an ultra-low-carbon steel from non-deoxidized or slightly deoxidized molten steel prepared by a steel making furnace, particularly a combined blowing converter or an LD converter, without shortening the life of the production apparatus.
2. Description of the Related Art
A continuous annealing apparatus, which has become available in recent years, has created a remarkable increase of the productivity of cold-rolled steel strips. This continuous annealing system has given a rise to the demand for ultra-low-carbon steel having a carbon content of 10 ppm or less.
Conventionally, an ultra low-carbon steel has been produced by decarburizing a molten steel until the carbon content is reduced to 0.02 to 0.05 wt% by using a converter, and then further decarburizing the steel under a reduced pressure by a vacuum degasifier such as an RH degasifier.
The conventional decarburizing method which utilizes a vacuum degasifier, however, could not produce ultra-low-carbon steel having a carbon content [C] less than 10 ppm in an industrial scale, because the decarburization rate is drastically decreased when the carbon content [C] has been reduced to a level less than 50 ppm.
For the purpose of enhancing the decarburization rate, it has been considered significant to increase the area of the reaction site. With this knowledge, it has been attempted to enhance the reaction rate by increasing the area of the reaction site area. Gas bubbles in molten steel or, surface of the molten steel in a vacuum chamber, or splash metal in the vacuum chamber is considered reaction site. It is still unclear, however, what degrees of contribution are made by these means to increase the reaction site area. Conventionally, it has been a common recognition that the above-mentioned three reaction sites will be increased by increasing the rate of supply of the Ar gas for agitation or recirculation. With this knowledge, it has been attempted to supply an RH degasifier with Ar gas at a large rate of 20 Nm.sup.3 /min or so.
Blowing of Ar gas at such a large rate, however, causes a problem in that the degasifier cannot operate continuously due to deposition of splash metal to the inner surface of the vacuum chamber of the vacuum degasifier as a result of vigorous generation of splash metal caused by the blowing of Ar gas.
In order to obviate the above-described problem, a method has been proposed and used in which hydrogen gas or a hydrogen-containing gas is blown into a molten steel so as to increase the content of hydrogen dissolved in the molten steel [H]. According to this method, a reaction expressed by 2H.fwdarw.H.sub.2 takes place to generate bubbles of hydrogen gas so as to enhance the effect of agitation and to increase the decarburization rate by the increase in the area of the reaction sites. This method is disclosed in Japanese Patent Laid-Open No. 57-194206.
It has been confirmed that this method can increase the decarburization rate in the low carbon region and, hence, contributes to improvement in the efficiency of production of ultra-low-carbon steel. This method, however, requires that the hydrogen content is maintained at a sufficiently high level, e.g., 3 to 5 ppm, in order to provide an appreciable effect in promoting decarburization. To maintain such a high hydrogen content, it has been required that hydrogen is blown at a rate not smaller than 5 Nm.sup.3 /min, when an RH degasifier having a capacity of, for example, 250 tons is used. This causes various problems such as an increase in the rate of generation of splash metal in the vacuum chamber, and shortening of the life of gas-blowing tuyere.
Moreover, the rate of utilization of hydrogen decreases, as the rate of hydrogen gas increases through the tuyeres provided in the side of the wall of recirculation pipe. Hence, it has been very difficult to maintain hydrogen content at such a high level by injecting hydrogen gas through the tuyeres provided in the side of the wall of the recirculating pipe.